Had a quick look, and found this article from Massey University in New Zealand - so I guess it's a 'cold' trial. Didn't read it, but it looks like it gives results for removal of various factors, so might be useful.

Don't know much about FTW specifically, and don't know what leads you to think that 2x would be a good scale factor, but could you compare conventional wetland treatment in cold/tropical climates, and use a similar scale factor?

My "quick and dirty", maybe educated guessing is based only on "warm & cold" anaerobic experienced made on ABR... I know, not much similar.

During "cold" winter time nutrient uptake by floating plants will be "0", but "The Biofilm" will still do a "job", more or less similar to ordinary aerobic CW's.

Maybe I could use roughly as a start and comparison less needed CW- m2/p.e. for the tropics?

Maybe mentioned reference: Van de Moortel A.M.K., Meers E., De Pauw N., Tack F.M.G. (2010). Effects of vegetation, season and temperature on the removal of pollutants in experimental floating treatment wetlands. Water, Air and Soil Pollution 212:281-297. will indicate more, but I do not have access and it is just based on a laboratory tests. I prefer real rough conditions